1. Field of the Invention
The present invention relates to an alternating-current power supply device that can improve a power factor of alternating-current load. More specifically, the invention relates to the alternating-current power supply device where a bi-directional magnetic energy recovery switch is inserted in series with the alternating-current load so as to function as a phase advancing capacitor instead of a conventional phase advancing capacitor and low-cost and miniaturization are realized.
2. Description of the Related Art
In the case where an alternating current is applied to an inductive load, it is necessary to apply a voltage higher than that of a direct-current resistance (resistance voltage at stationary state) at the time of start-up, and thus a power factor becomes poor due to a delay of a phase of current.
However, in order to improve the power factor of the alternating current, a phase advancing capacitor is inserted in series so that reactance is reduced, and the current is increased.
The improvement of the power factor by inserting the phase advancing capacitor in series is not suitable for the case where the power factor of the load changes according to an operating state or a circuit where a frequency of power source greatly changes.
On the other hand, the power factor of the load changes at the time from the start-up to the stationary operation like induction motors. In order to compensate this, however, an inverter-converter set of AC-DC-AC link using a semiconductor switch is used, and thus a frequency and a voltage are reduced at the time of start-up and the number of revolution is controlled at the time of operation. However, since the cost is high, harmonics distortion occurs due to PWM control and a large electrolytic capacitor is necessary for maintaining a DC voltage, the entire size becomes large.
At the time of the start-up of the induction motor, an inductance component is large, the power factor of the current is poor, and an active current is low. For this reason, a large torque (starting torque) cannot be obtained at the time of start-up.
Such a conventional method of improving the power factor has a limitation, and thus an alternating-current power supply device which has a power factor improving circuit instead of using the phase advancing capacitor and realizes low cost and miniaturization is requested.